Credit

Caption

Interference between two HeNe laser beams. A Helium-Neon laser beam is split into two beams which are then crossed at a very small angle. A screen is placed perpendicular to the beams' propagation direction, in the region where the beams intersect. Because the two beams are coherent with each other, characteristic spatial modulation of light intensity, called interference fringes, is observed and photographed on the screen. Conceptually, this is a modern-day replication of Thomas Young's double-slit experiment. Characteristic laser speckle pattern is also observed. The speckle results from scattering of the monochromatic beam by the screen, which is rough on a microscopic (wavelength) scale.